Landfall — Whenever a land enters the battlefield under your control, tap target creature an opponent controls. If that land is an Island, that creature doesn't untap during its controller's next untap step.

Guardian of Tazeem’s landfall ability can target a creature that’s already tapped. If the land that caused it to trigger was an Island, that creature won’t untap during its controller’s next untap step.

8/25/2015

The ability tracks the creature, but not its controller. If the creature changes controllers before its first controller’s next untap step has come around, then it won’t untap during its new controller’s next untap step.

8/25/2015

A landfall ability triggers whenever a land enters the battlefield under your control for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

8/25/2015

When a land enters the battlefield under your control, each landfall ability of the permanents you control will trigger. You can put them on the stack in any order. The last ability you put on the stack will be the first one to resolve.

8/25/2015

If the ability has an additional or replacement effect that depends on the land having a certain basic land type, the ability will check that land’s type as the ability resolves. If, at that time, the land that entered the battlefield is no longer on the battlefield, use its types when it left the battlefield to determine what happens.